In processing semiconductor devices in the microelectronics industry, wire bonding is a widely used, well-established method of chip interconnection with external circuitry. FIG. 1 shows a side view of a conventional apparatus for forming wire bonds. In a conventional wire bonding process used with interposer or other carrier substrates, the back side, or non-active side, of a semiconductor die or chip 10, such terms being used interchangeably in the industry, is firmly attached to a suitable substrate or package bottom 20. The chip 10 is conventionally bonded to the substrate 20 using either an organic adhesive, a glass, or a metal-alloy reflow process generally shown as adhesive layer 30. Additional chips 10 may be subsequently attached on top of the first chip 10 using additional adhesive layers 30 between each chip 10 of the resulting chip stack. The chip 10 and substrate 20 are positioned on a processing block 40 of a wire bonding apparatus in contact with either the die back for a board-on-chip (BOC) confirmation or the substrate for a chip-on-board (COB) configuration. A wire bonding operation is then carried out, wherein conductive wires 50 are extended between and fused at each end thereof to bond pads 60 on chip 10 and to bonding sites 70 on the surface of the substrate 20 by a wire bonding capillary, typically using thermocompression bonding, ultrasonic bonding, or a combination of ultrasonic and heat in combination with compression, sometimes referred to as “thermosonic bonding.”
In order to raise the temperature of the bond pads 60 and bonding sites 70 to an operating temperature wherein wire bonding may be rapidly and reliably affected, the processing block 40 is conventionally heated. In such a configuration, processing block 40 is typically known as a “heater” block, which conductively heats all of the substrate 20 and the chip 10, including the bonding pads 60 and bonding sites 70. The wire bonding operating temperature varies depending on the specific application. For example, the wire bonding operating temperature for thin small-outline packages (TSOPs) may be generally between 200° C. and 230° C., while for fine ball grid array (FBGA) packages the wire bonding operating temperature may generally be about 150° C.
Heating the chip to the elevated temperatures necessary to adequately heat the bonding pads may potentially damage the chip by overheating. Indeed, the elevated temperatures introduce thermo-mechanical stress on the active die surface and the integrated circuitry thereof. With the continuous reduction in size and thickness of semiconductor chips in order to meet packaging requirements, adverse effects of these thermo-mechanical stresses are significantly increased when such a relatively fragile chip is heated. Moreover, when chips are stacked one on top of another, as in a stacked, multi-chip package (MCP), the ambient temperature at the top chip layer is significantly lower than the ambient temperature at the lower layer or layers when the sole preheating source is the processing block, due to the thermal gradient of the chip stack. Thus, semiconductor chips in the lower layer or layers must be subjected to undesirably high temperatures in order for the bond pads in a higher layer or layers of chips to reach the required wire bonding operating temperatures. The increased temperature in the lower layer or layers introduces even more thermo-mechanical stresses in those lower layers.
Furthermore, the material used in adhesive layer 30 to attach one or more chips 10 to the substrate 20 may in some instances have voids, or air pockets, when it is applied. This is the case with so-called “skip cure” adhesives, which are also termed “b-stage” adhesives and are desirably not fully cured until the chip package is encapsulated, as in a transfer molding process. Such voids may generally be driven out during encapsulation of the semiconductor chip or chips and at least a portion of the substrate 20 when such an adhesive material between a chip and the substrate or between two stacked chips is subjected to the high molding pressures common to such transfer molding processes. However, if such an adhesive material is exposed to excessively high temperatures for an extended duration, or repeatedly, prior to encapsulation, premature cross-linking of the adhesive material takes place, which permanently traps the voids. The trapped voids may cause the device to later fail, exhibiting the so-called “popcorn” effect wherein the gas trapped in the voids expands and compromises the encapsulant envelope. In addition, repeated heating for wire-bonding a multi-chip stack may cause the adhesive material and substrate to outgas contaminants onto the in-process package, which may adversely affect downstream assembly processes
Some approaches have been developed to heat the bond pads and bonding sites in addition to, or as an alternative to, using a heater block. Several of these approaches use a flood-type infrared radiation source to heat the semiconductor die and the substrate from above instead of from below. However, shining such an infrared radiation onto the entire top surface or even a substantial portion of the top surface of the semiconductor die still undesirably subjects a substantial portion, if not the entire die, to heating. Such generalized application of heat subjects the active die surface and the integrated circuitry thereof to the same undesirable heat-induced thermo-mechanical stresses that are caused by heating from the bottom using a heater block and may, as with the prolonged or repeated use of a heater block, prematurely cross-link the die-attach adhesive material and cause undesirable outgassing from the adhesive material and the substrate.
In other approaches, a laser is used to heat a single bond pad from above. However, using a single laser requires that each bond pad be heated and immediately bonded. Such a process requires a multiplicity of steps including turning on the laser, heating the bond pad, turning the laser off, bonding the wire, moving the laser to a subsequent bond pad, turning the laser on, heating the bond pad, turning the laser off, bonding the wire, etc. Such a method of heating the pad then bonding, heating the next pad then bonding, etc., requires many steps and may slow down the wire bonding method. In addition, if the laser is mispositioned just a little from the target bond pad, the laser may radiate the semiconductor die, causing damage to the integrated circuitry in the semiconductor die from the intense heat emitted by the laser.
To enable the manufacturing of wire-bonded semiconductor devices without subjecting these devices to potentially damaging heat while also protecting the adhesive used to attach the chip to the substrate from premature cross-linking, it would be desirable to develop a wire bonding method and apparatus for effectively limiting areas heated on a chip to substantially those areas including the bond pads and without subjecting an entire chip to substantial and repeated heating.